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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Pathway structure determination in complex stochastic networks with non-exponential dwell times

Analysis of complex networks has been widely used as a powerful tool for investigating various physical, chemical, and biological processes. To understand the emergent properties of these complex systems, one of the most basic issues is to determine the structure and topology of the underlying networks. Recently, a new theoretical approach based on first-passage analysis has been developed for investigating the relationship between structure and dynamic properties for network systems with exponential dwell time distributions. However, many real phenomena involve transitions with non-exponential waiting times. We extend the first-passage method to uncover the structure of distinct pathways in complex networks with non-exponential dwell time distributions. It is found that the analysis of early time dynamics provides explicit information on the length of the pathways associated to their dynamic properties. It reveals a universal relationship that we have condensed in one general equation, which relates the number of intermediate states on the shortest path to the early time behavior of the first-passage distributions. Our theoretical predictions are confirmed by extensive Monte Carlo simulations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Probing plasmonic breathing modes optically

The confinement of surface plasmon modes in flat nanoparticles gives rise to plasmonic breathing modes. With a vanishing net dipole moment, breathing modes do not radiate, i.e., they are optically dark. Having thus escaped optical detection, breathing modes were only recently revealed in silver nanodisks with electron energy loss spectroscopy in an electron microscope. We show that for disk diameters >200 nm, retardation induced by oblique optical illumination relaxes the optically dark character. This makes breathing modes and thus the full plasmonic mode spectrum accessible to optical spectroscopy. The experimental spectroscopy data are in excellent agreement with numerical simulations.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Simple adaptive control system design for a quadrotor with an internal PFC

The paper deals with an adaptive control system design problem for a four rotor helicopter or quadrotor. A simple adaptive control design scheme with a parallel feedforward compensator (PFC) in the internal loop of the considered quadrotor will be proposed based on the backstepping strategy. As is well known, the backstepping control strategy is one of the advanced control strategy for nonlinear systems. However, the control algorithm will become complex if the system has higher order relative degrees. We will show that one can skip some design steps of the backstepping method by introducing a PFC in the inner loop of the considered quadrotor, so that the structure of the obtained controller will be simplified and a high gain based adaptive feedback control system will be designed. The effectiveness of the proposed method will be confirmed through numerical simulations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

On resistive magnetohydrodynamic studies of sawtooth oscillations in tokamaks

A fundamental requirement for the validity and accuracy of any large-scale computation is sufficiently well-resolved length and time scales relevant to the problem under study. Ironically, despite the enormous computational resources available today, poorly resolved length scales in sophisticated nonlinear calculations are not uncommon. Using the internal kink mode that is responsible for tokamak sawtooth oscillations as an example, consequences of not resolving in sufficient detail the linear and nonlinear layer widths of the resistive n = 1 mode and its nonlinear spectrum are examined. Poor radial and spectral resolution are shown to cause nonphysical, large-scale stochasticity that can be erroneously associated with a fast temperature collapse and sawtooth crash. With the assistance of a nonlinear mode coupling model, a sufficiently well-resolved toroidal spectrum is shown to require at least an order of magnitude more toroidal modes than is commonly used at dissipation levels relevant to today's tokamaks. A subgrid-scale model is introduced that helps with the spectral resolution problem by reducing the required number of degrees of freedom from that of a well-resolved direct numerical simulation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Evolution of the angular distribution of laser-generated fast electrons due to resistive self-collimation

The evolution of the angular distribution of laser-generated fast electrons propagating in dense plasmas is studied by 3D numerical simulations. As resistively generated magnetic fields can strongly influence and even pinch the fast electron beam, the question of the effect on the angular distribution is of considerable interest. It was conjectured that in the limit of strong collimation, there will only be minimal changes to the angular distribution, whereas the largest reduction in the angular distribution will occur where there is only modest pinching of the fast electron beam and the beam is able to expand considerably. The results of the numerical simulations indicate this conjecture.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Coherent control of magnetization precession in electrically detected time domain ferromagnetic resonance

We demonstrate coherent control of time domain ferromagnetic resonance by all electrical excitation and detection. Using two ultrashort magnetic field steps with variable time delay we control the induction decay in yttrium iron garnet (YIG). By setting suitable delay times between the two steps the precession of the magnetization can either be enhanced or completely stopped. The method allows for a determination of the precession frequency within a few precession periods and with an accuracy much higher than can be achieved using fast fourier transformation. Moreover it holds the promise to massively increase precession amplitudes in pulsed inductive microwave magnetometry (PIMM) using low amplitude finite pulse trains. Our experiments are supported by micromagnetic simulations which nicely confirm the experimental results.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Effect of surface roughness and size of beam on squeeze-film damping—Molecular dynamics simulation study

We use large-scale molecular dynamics (MD) to characterize fluid damping between a substrate and an approaching beam. We focus on the near contact regime where squeeze film (where fluid gap is comparable to the mean free path of the gas molecules) and many-body effects in the fluid become dominant. The MD simulations provide explicit description of many-body and non-equilibrium processes in the fluid as well as the surface topography. We study how surface roughness and beam width increases the damping coefficient due to their effect on fluid mobility. We find that the explicit simulations are in good agreement with prior direct simulation Monte Carlo results except at near-contact conditions where many-body effects in the compressed fluid lead the increased damping and weaker dependence on beam width. We also show that velocity distributions near the beam edges and for short gaps deviate from the Boltzmann distribution indicating a degree of local non-equilibrium. These results will be useful to parameterize compact models used for microsystem device-level simulations and provide insight into mesoscale simulations of near-contact damping.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

MECHANISM OF OUTFLOWS IN ACCRETION SYSTEM: ADVECTIVE COOLING CANNOT BALANCE VISCOUS HEATING?

Based on the no-outflow assumption, we investigate steady-state, axisymmetric, optically thin accretion flows in spherical coordinates. By comparing the vertically integrated advective cooling rate with the viscous heating rate, we find that the former is generally less than 30% of the latter, which indicates that the advective cooling itself cannot balance the viscous heating. As a consequence, for radiatively inefficient flows with low accretion rates such as M-dot ≲10{sup −3} M-dot {sub Edd}, where M-dot {sub Edd} is the Eddington accretion rate, the viscous heating rate will be larger than the sum of the advective cooling rate and the radiative cooling one. Thus, no thermal equilibrium can be established under the no-outflow assumption. We therefore argue that in such cases outflows ought to occur and take away more than 70% of the thermal energy generated by viscous dissipation. Similarly, for optically thick flows with extremely large accretion rates such as M-dot ≳10 M-dot {sub Edd}, outflows should also occur owing to the limited advection and the low efficiency of radiative cooling. Our results may help to understand the mechanism of outflows found in observations and numerical simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Estimating statistical isotropy violation in CMB due to non-circular beam and complex scan in minutes

Mild, unavoidable deviations from circular-symmetry of instrumental beams along with scan strategy can give rise to measurable Statistical Isotropy (SI) violation in Cosmic Microwave Background (CMB) experiments. If not accounted properly, this spurious signal can complicate the extraction of other SI violation signals (if any) in the data. However, estimation of this effect through exact numerical simulation is computationally intensive and time consuming. A generalized analytical formalism not only provides a quick way of estimating this signal, but also gives a detailed understanding connecting the leading beam anisotropy components to a measurable BipoSH characterisation of SI violation. In this paper, we provide an approximate generic analytical method for estimating the SI violation generated due to a non-circular (NC) beam and arbitrary scan strategy, in terms of the Bipolar Spherical Harmonic (BipoSH) spectra. Our analytical method can predict almost all the features introduced by a NC beam in a complex scan and thus reduces the need for extensive numerical simulation worth tens of thousands of CPU hours into minutes long calculations. As an illustrative example, we use WMAP beams and scanning strategy to demonstrate the easability, usability and efficiency of our method. We test all our analytical results against that from exact numerical simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Scalar and vector perturbations in a universe with discrete and continuous matter sources

We study a universe filled with dust-like matter in the form of discrete inhomogeneities (e.g., galaxies and their groups and clusters) and two sets of perfect fluids with linear and nonlinear equations of state, respectively. The background spacetime geometry is defined by the FLRW metric. In the weak gravitational field limit, we develop the first-order scalar and vector cosmological perturbation theory. Our approach works at all cosmological scales (i.e. sub-horizon and super-horizon ones) and incorporates linear and nonlinear effects with respect to energy density fluctuations. We demonstrate that the scalar perturbation (i.e. the gravitational potential) as well as the vector perturbation can be split into individual contributions from each matter source. Each of these contributions satisfies its own equation. The velocity-independent parts of the individual gravitational potentials are characterized by a finite time-dependent Yukawa interaction range being the same for each individual contribution. We also obtain the exact form of the gravitational potential and vector perturbation related to the discrete matter sources. The self-consistency of our approach is thoroughly checked. The derived equations can form the theoretical basis for numerical simulations for a wide class of cosmological models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Nuclear signal simulation applied to gas ionizing chambers

Particle transport codes used in detector simulation allow the calculation of the energy deposited by charged particles produced following an interaction. The pulses temporal shaping is more and more used in nuclear measurement into pulse shape analysis techniques. A model is proposed in this paper to simulate the pulse temporal shaping and the associated noise level thanks to the output track file PTRAC provides by Monte-Carlo particle transport codes. The model has been dedicated to ion chambers and more especially for High Pressure Xenon chambers HPXe where the pulse shape analysis can resolve some issues regarding with this technology as the ballistic deficit phenomenon. The model is fully described and an example is presented as a validation of such full detector simulation. (authors)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

MO-FG-CAMPUS-IeP2-01: Characterization of Beam Shaping Filters and Photon Spectra From HVL Profiles in CT

Purpose: Advanced dosimetry in CT (e.g. the Monte Carlo method) requires an accurate characterization of the shaped filter and radiation quality used during a scan. The purpose of this work was to develop a method where half value layer (HVL) profiles along shaped filters could be made. From the HVL profiles the beam shaping properties and effective photon spectrum for a particular scan can be inferred. Methods: A measurement rig was developed to allow determinations of the HVL under a scatter-free narrow-beam geometry and constant focal spot to ionization chamber distance for different fan angles. For each fan angle the HVL is obtained by fitting the transmission of radiation through different thicknesses of an Al absorber (type 1100) using an appropriate model. The effective Al thickness of shaped filters and effective photon spectra are estimated using a model of photon emission from a Tungsten anode. This method is used to obtain the effective photon spectra and effective Al thickness of shaped filters for a CT scanner recently introduced to the market. Results: This study resulted in a set of effective photon spectra (central ray) for each kVp along with effective Al thicknesses of the different shaped filters. The effective photon spectra and effective Al thicknesses of shaped filters were used to obtain numerically approximated HVL profiles and compared to measured HVL profiles (mean absolute percentage error = 0.02). The central axis HVL found in the vendor’s technical documentation were compared to approximated HVL values (mean absolute percentage error = 0.03). Conclusion: This work has resulted in a unique method of measuring HVL profiles along shaped filters in CT. Further the effective photon spectra and the effective Al thicknesses of shaped filters that were obtained can be incorporated into Monte Carlo simulations.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2018

Anomalous Diffusion Equations with Multiplicative Acceleration

A generalization of the model of Lévy walks with traps is considered. The main difference between the model under consideration and the already existing models is the introduction of multiplicative particle acceleration at collisions. The introduction of acceleration transfers the consideration of walks to coordinate–momentum phase space, which allows both the spatial distribution of particles and their spectrum to be obtained. The kinetic equations in coordinate–momentum phase space have been derived for the case of walks with two possible states. This system of equations in a special case is shown to be reduced to ordinary Lévy walks. This system of kinetic equations admits of integration over the spatial variable, which transfers the consideration only to momentum space and allows the spectrum to be calculated. An exact solution of the kinetic equations can be obtained in terms of the Laplace–Mellin transform. The inverse transform can be performed only for the asymptotic solutions. The calculated spectra are compared with the results of Monte Carlo simulations, which confirm the validity of the derived asymptotics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Monte Carlo Simulation of a Segmented Detector for Low-Energy Electron Antineutrinos

Detection of low-energy electron antineutrinos is of importance for several purposes, such as ex-vessel reactor monitoring, neutrino oscillation studies, etc. The inverse beta decay (IBD) is the interaction that is responsible for detection mechanism in (organic) plastic scintillation detectors. Here, a detailed study will be presented dealing with the radiation and optical transport simulation of a typical segmented antineutrino detector withMonte Carlo method using MCNPX and FLUKA codes. This study shows different aspects of the detector, benefiting from inherent capabilities of the Monte Carlo simulation codes.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2017

The Fixed Target Experiment for Studies of Baryonic Matter at the Nuclotron (BM@N)

BM@N (Baryonic Matter at Nuclotron) is the first experiment to be realized at the NICA-Nuclotron accelerator complex. The aim of the BM@N experiment is to study relativistic heavy ion beam interactions with fixed targets. The BM@N setup, results of Monte Carlo simulations, and the BM@N experimental program are presented.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

High-Fidelity Space-Time Adaptive Multiphysics Simulations in Nuclear Engineering

We delivered a series of fundamentally new computational technologies that have the potential to significantly advance the state-of-the-art of computer simulations of transient multiphysics nuclear reactor processes. These methods were implemented in the form of a C++ library, and applied to a number of multiphysics coupled problems relevant to nuclear reactor simulations.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2016

Dark Energy Studies with LSST Image Simulations, Final Report

This grant funded the development and dissemination of the Photon Simulator (PhoSim) for the purpose of studying dark energy at high precision with the upcoming Large Synoptic Survey Telescope (LSST) astronomical survey. The work was in collaboration with the LSST Dark Energy Science Collaboration (DESC). Several detailed physics improvements were made in the optics, atmosphere, and sensor, a number of validation studies were performed, and a significant number of usability features were implemented. Future work in DESC will use PhoSim as the image simulation tool for data challenges used by the analysis groups.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Time-dependent Models of Magnetospheric Accretion onto Young Stars

Accretion onto Classical T Tauri stars is thought to take place through the action of magnetospheric processes, with gas in the inner disk being channeled onto the star’s surface by the stellar magnetic field lines. Young stars are known to accrete material in a time-variable manner, and the source of this variability remains an open problem, particularly on the shortest (∼day) timescales. Using one-dimensional time-dependent numerical simulations that follow the field line geometry, we find that for plausibly realistic young stars, steady-state transonic accretion occurs naturally in the absence of any other source of variability. However, we show that if the density in the inner disk varies smoothly in time with ∼day-long timescales (e.g., due to turbulence), this complication can lead to the development of shocks in the accretion column. These shocks propagate along the accretion column and ultimately hit the star, leading to rapid, large amplitude changes in the accretion rate. We argue that when these shocks hit the star, the observed time dependence will be a rapid increase in accretion luminosity, followed by a slower decline, and could be an explanation for some of the short-period variability observed in accreting young stars. Our one-dimensional approach bridges previous analytic work to more complicated multi-dimensional simulations and observations.

79 ASTRONOMY AND ASTROPHYSICS↗